DDI DESIGNER

National DDI expert and advocate Gilbert Chlewicki will give the keynote address at the UDOT Research Workshop on May 10.

Gilbert Chlewicki

Chlewicki developed the design for the Diverging Diamond Interchange as a graduate student. After designing the innovative interchange, he visited France and on a bus tour, was surprised to see that the design was already in existence. While disappointed that he was not the first, he continued to work on the DDI. Chlewicki has pioneered other designs as well, including the Continuous T Intersection, which he will discuss in detail at the workshop.

As a forward to his keynote address at the workshop, he answered some questions about non-traditional transportation designs.

What prompted you to want to develop non-traditional designs?

“I’ve been designing roads since I was a little kid. I used to draw long stretches of highway on old dot-matrix computer paper that had each sheet of paper attached to the next. I used to make the lanes wide enough for my hot wheels to drive on my roads. The drawings had full signing and traffic controls. When I would finish a drawing, I would stretch it out across the house and have my younger sisters drive on them to find certain destinations. In high school, I made my lanes narrower so that I could fit more into a drawing and eventually created a new highway (which I called I-74) that went from New Jersey, through southern Pennsylvania, Ohio and finally to Indiana.

My drawings were always in pen, so when I would make a “mistake”, I would incorporate the mistake into the final design. Anytime I drew a more complex design, I would always try to figure out how both the geometrics and traffic operations would work and then think about what I could do to improve the design for the future. I came up with several designs on my own before I even knew they existed such as a roundabout and a single point urban interchange (SPUI). There were other designs that I still believe are original concepts. So I’ve been developing unique geometric designs for nearly 30 years now and am still continuing to come up with new designs.”

Are transportation agencies/engineers initially resistant to the DDI or other non-traditional designs? Why, and what has to happen for that resistance to be dropped?

“There has been a lot of resistance to non-traditional designs. Just as an example, the modern roundabout has been around

An aerial photo of I-44 / Kansas Expressway Diverging Diamond Interchange in Springfield, Missouri. First of its kind in the U.S. Photo from Missouri Department of Transportation.

nearly 50 years but it has only gained partial acceptance in the US over the past 10-15 years even though it is a great design. There are over 40 unique documented non-traditional designs but about half of these designs have never been constructed even though they show promise. Of the 20+ designs that have been constructed, many of those you can only find sporadically in the country (ex. one echelon in Florida) or prominently in one state (ex. the jughandle in New Jersey, the median u-turn in Michigan).

The DDI has been very fortunate to gain acceptance very quickly across a lot of the transportation world and my hope is that the DDI will be the main example that helps stimulate building other non-traditional designs. However, there is still resistance to the DDI in a large part of the transportation world.”

“There are many different reasons for the resistance of non-traditional designs. Some are due to not wanting to change when a more well-established design could work almost as well. Some are due to consultants not thinking that their clients will accept something different and “unproven”. Some don’t want to try something new. Some are worried about liability. Some are scared to fail. Some think that these designs cost too much. Some don’t even know that these designs exist and most of those that know they exist don’t understand which design is best. And some are scared of the politics and/or community acceptance.

I think three big things need to happen for this resistance to change. One, we need to educate every one of these designs. There’s enough information out there now and the more people know and understand, the more willing they will be to try these designs. Two, we need champions of non-traditional designs in both the consulting world and in transportation agencies. When we have the right people promoting these designs, the resistance will begin to fall. And three, we need to create a more innovative culture in the transportation industry. When we promote innovation, we will be more likely to accept it.”

How has the driving public accepted DDIs?

“For the most part, the public has been very accepting of DDIs, perhaps even more accepting than the transportation industry. When the public sees how easy it is to navigate in the DDI and then they see the benefits of the design in terms of the traffic operations, costs, and safety improvements, it becomes a no-brainer to most of the public.

I worked on a planning project back in 2005, where we had a DDI as one of our options. This might have been the first project that was considering a DDI in the country. All of the alternatives had the interchange operating at a LOS A or B in the design year, so there was little benefit from an operations point of view of the DDI versus any other alternative. The public still voted for the DDI over the other alternatives. Ultimately the DDI was not selected because the agency did not think it was the proper location to put the first DDI in the state and since it was a new interchange, the DDI did not show any inherent benefit over the other alternatives in terms of traffic operations or costs.”

In your opinion, what is the future of the DDI in the US?

“The future is extremely bright for the DDI in the US. I wrote a paper that was published in Transportation Research Record last year that showed that the DDI will likely be the best option for a multi-lane arterial of any diamond form regardless of turning movements, when considering costs and traffic operations. I think the DDI will replace the need to build any more SPUIs, except in a few situations where it might be better geometrically.

In my opinion, the DDI should be considered as an alternative for all interchange projects because of the many benefits of the design. It may not be the best solution in every case, but it has enough benefits to merit consideration. We have a similar policy in Maryland concerning roundabouts. All projects in Maryland must consider a roundabout for any intersection improvement.

I can easily see there being over a hundred DDIs across the country by the end of the decade. And when you consider that the DDI on average costs roughly $10 Million less than the next best alternative, the country should be saving over $1 Billion in transportation costs!”

Your new Continuous T sounds interesting! Can you send an explanation and maybe refer me to some information online?

“The Two-Way Continuous-T Intersection allows non-stop flow of thru traffic in both directions of a T-Intersection without any grade-separation, while at the same time vastly increasing the percentage of green time for the left turn movements. How is this possible? The design takes concepts from three other innovative intersections (the Continuous-T Intersection, Continuous Flow Intersection, and Double-Wide Intersection) to create this new design. You can get more information on this design from the materials in the 2012 TRB annual meeting where you can find my paper and poster that I presented. If time permits, I will be talking about this design during the keynote address as an example of how innovation is developed.”